1. Field of the Invention
The present invention relates to a color filter substrate for a liquid crystal display device and more particularly relates to a color filter substrate including columnar spacers. The present invention also relates to a liquid crystal display device including such a color filter substrate.
2. Description of the Related Art
In color LCDs used extensively today, a color filter is provided for each and every picture element (dot). Typically, three types of color filters for the three primary colors of light (i.e., red (R), green (G) and blue (B)) are arranged in a predetermined pattern for respective picture elements. In this case, a set of three picture elements (dots) provided with the R, G and B color filters form a single pixel, which can be the smallest unit to conduct a color display operation.
However, the colors of color filters (or picture elements) to be used as a single set do not have to be R, G and B but may also be a combination of cyan (C), magenta (M) and yellow (Y) or a set of any other suitable colors. Also, as is well known in the art, a striped arrangement, a delta arrangement or a mosaic arrangement may be adopted for those color filters.
In an LCD, a liquid crystal layer is usually arranged between two substrates, one of which includes a color filter layer where a plurality of color filters are provided for respective picture elements. For example, in a TFT LCD, a liquid crystal layer is interposed between a TFT substrate on which picture element electrodes, TFTs and other circuit components are arranged and a counter substrate including a counter electrode and a color filter layer thereon. An LCD including a color filter layer on a TFT substrate is also known. However, most of LCDs currently on the market include a color filter layer on the counter substrate. That is why such a counter substrate with a color filter layer is often called a “color filter substrate”.
To control the thickness of a liquid crystal layer (which is also called a “cell gap”) in bonding a color filter substrate and a TFT substrate together, spacers are provided. However, as the display quality of LCDs has improved more and more significantly, deterioration in display quality due to the presence of those spacers has become an increasingly serious issue.
Specifically, in the prior art, bead-like or rod-like spacers with a predetermined diameter are scattered on the surface of a color filter substrate or a TFT substrate. That is why it is difficult to arrange those spacers at a uniform density over the entire display screen. As a result, the cell gap may vary significantly from one position to another or the spacers may collect together locally, thus sometimes causing defects in display. Also, when those spacers are arranged inside a picture element, the aperture ratio of the LCD may decrease substantially or those spacers may be sensed as bright spots to the viewer's eyes.
In view of these considerations, a method of selectively arranging the spacers in a predetermined area outside of each picture element (which is typically an area shielded from external light with a black matrix) was developed. For example, a method of forming columnar spacers in such a predetermined area by a photolithographic process using a photosensitive resin (which is often called a “photoresist”) has been actually used in manufacturing facilities.
The deterioration in display quality as mentioned above can be minimized by controlling the cell gap with columnar spacers. However, to further improve the display quality, various methods of arranging or forming those columnar spacers have been proposed.
For instance, Japanese Patent Application Laid-Open Publication No. 2003-84289 discloses techniques for minimizing production of bubbles in a liquid crystal layer at a low temperature and increasing the withstand load thereof by providing two types of columnar spacers of mutually different heights on a color filter substrate.
FIGS. 21A and 21B illustrate a color filter substrate 70 disclosed in Japanese Patent Application Laid-Open Publication No. 2003-84289. As shown in FIG. 21A, this color filter substrate 70 includes columnar spacers 76 and 77, which are arranged outside of picture elements. In the area outside of the picture elements on the color filter substrate 70, a black matrix 72, color filters 73, 74, and a common electrode 75 are stacked one upon the other on a transparent substrate 71 and the columnar spacers 76 and 77 are arranged thereon as shown in FIG. 21B.
The color filter 73 and its adjacent color filter 74 have mutually different thicknesses, and therefore, the columnar spacers 76 and 77 arranged on the color filters 73 and 74, respectively, have different heights.
Generally speaking, in an LCD that uses columnar spacers, if the density of columnar spacers (i.e., the number of columnar spacers per unit area) were increased to enhance the withstand load thereof, then it would be more and more difficult for the cell gap to catch up with the shrinkage of a liquid crystal layer that could occur at a low temperature. As a result, bubbles would be produced in the liquid crystal layer (which phenomenon will be referred to herein as “low-temperature bubbling”).
If the two types of columnar spacers 76 and 77 with different heights were arranged such that the cell gap is controllable with only the higher columnar spacers 76 in most cases as disclosed in Japanese Patent Application Laid-Open Publication No. 2003-84289, then the effective spacer density would be defined by only the higher columnar spacers 76. In that case, the cell gap could catch up with the shrinkage of the liquid crystal layer more easily. Also, if the cell gap decreased upon the application of load to the liquid crystal panel, the two substrates would be supported by both the higher and lower columnar spacers 76 and 77 alike (and the effective spacer density would be defined by both of the two types of columnar spacers 76 and 77 in that case). Consequently, high withstand load would be realized.
Furthermore, to realize an even more uniform cell gap, the columnar spacers are preferably provided in not just the display area but also a non-display area surrounding the display area. However, different structures are defined in the display area and non-display area on the substrates. For that reason, it is difficult to control the heights of the columnar spacers to their best value(s) (e.g., equalize their heights with each other) in both the display area and non-display area.
Japanese Patent Application Laid-Open Publication No. 2001-51266 discloses a technique for forming a multilayer structure of a black matrix and color filters in a non-display area and arranging columnar spacers on the multilayer structure. According to this technique, by adjusting the number of layers included in the multilayer structure, the height of the columnar spacers arranged in the non-display area can be controlled. Thus, the columnar spacers can have their height controlled differently in the display area and in the non-display area.
However, if the color filters 73 and 74 of mutually different thicknesses were used as disclosed in Japanese Patent Application Laid-Open Publication No. 2003-84289, then the liquid crystal layer would have a different thickness in some picture elements than in other picture elements. In that case, the magnitudes of retardation caused by the liquid crystal layer on light would be different in these two groups of picture elements. As a result, unwanted coloring would be produced and the display quality would decrease in a black display mode or in a grayscale display mode.
On the other hand, according to the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2001-51266, the heights of the columnar spacers are controlled by changing the numbers of layers in the multilayer structure. However, this technique just allows the designer to change the heights of the columnar spacers by no less than the thickness of any of the layers included in the multilayer structure. That is why the heights of the columnar spacers can be changed only discontinuously.
In sum, a sufficiently effective technique for controlling the heights of columnar spacers arbitrarily by a simple process has not been developed or discovered yet.